Synthesis gas, mixtures of carbon oxides and hydrogen, can be catalytically converted to a variety of organic compounds. Of particular interest in the industry is the selective synthesis of aliphatic alcohols containing 2 or greater carbon atoms (i.e. "C.sub.2 +" aliphatic alcohols). These compounds have a variety of chemical uses and have been shown to have beneficial properties when added to gasoline. Of particular interest is the production of branched primary alcohols such as isobutanol, which can be dehydrated selectively to isobutylene, a key chemical intermediate.
Processes for the production of mixtures of methanol and higher alcohols from synthesis gas are taught in the art. These processes are largely based on synthesis gas conversion in the presence of a heterogeneous catalyst in a packed bed reactor. However, because synthesis gas conversion to higher alcohols is highly exothermic, the use of a slurry reactor is advantageous. In a slurry reactor, a heterogeneous catalyst, in the powder from, is suspended in a liquid medium. The intimate contact of the liquid medium with the catalyst provides a more effective means of removing the heat of reaction from the catalyst. This feature allows isothermal reactor operation at higher synthesis conversion per reactor pass than that possible in a packed bed reactor. Isothermality enables good control of reaction selectivity while protecting the catalyst from damage by local overheating.
A variety of catalysts have been developed for the synthesis of mixtures of alcohols from synthesis gas. Generally, these catalysts consist of mixed metals and/or metal oxides, or metal sulfides. Since most of these catalysts tend to produce methanol as the dominant alcohol from the "C.sub.1 " based feedstock, selectivity to C.sub.2+ alcohols has been a key technical challenge. It is taught in the art that selectivity to C.sub.2+ alcohols, particularly branched isomers, is enhanced by the inclusion of an alkali metal from group 1A of the periodic table in the catalyst formulation. Moreover, since selectivity to C.sub.2+ alcohols is often maximized for a specific alkali content, the catalysts are often treated with a specific quantity of alkali in a controlled manner. Doping of the catalyst with alkali is usually done in a separate production step by impregnation/promotion with a solution of an alkali salt, followed by drying and, in some cases, calcination.
The objective of the present invention is to provide a simpler, more efficient method of producing an alkalized catalyst for the synthesis of C.sub.2+ aliphatic alcohols, and more particularly isobutanol, when the catalyst is used in a slurry reactor.
The state-of-the-art catalysts for the synthesis of mixtures of alcohols, with high selectivity to C.sub.2+ aliphatic alcohols, are mixed metals and/or metal oxides or metal sulfides which have been promoted with alkali metal. One class of these catalysts, based on copper and zinc oxide, have been shown to catalyze the synthesis of methanol and higher alcohols at high yield. Moreover, these catalysts have been shown to have a high selectivity to branched alcohols, like isobutanol, and a relatively low yield of paraffinic hydrocarbons, which are common undesirable byproducts.
Generally, copper and zinc oxide-based catalysts are produced by co-precipitation of copper, zinc, and optionally, other metals from solution by a base (commonly an alkali compound). Washing, drying and calcination of the precipitate produces the mixed oxide form. The dried and calcined catalyst is then impregnated with a solution of an alkali metal compound which contains the desired quantity of alkali, then dried and often calcined again. This procedure deposits a controlled quantity of alkali onto the catalyst surface. The catalyst is converted to the active form, which contains reduced copper, for synthesis gas conversion by reduction using a hydrogen-containing gas.
U.S. Pat. No. 4,598,061 by Schneider et al. describes a catalyst for the synthesis of methanol and higher alcohols which contains copper oxide, zinc oxide, aluminum oxide, and an alkali metal carbonate or oxide. This catalyst is formed by precipitation of copper and zinc from solution, by an alkali compound, in the presence of colloidally dispersed aluminum hydroxide. The precipitate is washed and calcined, then impregnated with an alkali metal compound and dried again. The preferred quantity of alkali metal is 13-130.times.10.sup.-6 gram atom per gram of copper oxide/zinc oxide/aluminum oxide precursor.
Smith and Anderson (The Canadian Journal of Chemical Engineering, Volume 61, February 1983) describe a Cu/ZnO catalyst which, when promoted with potassium carbonate, yields more higher alcohols, including isobutanol, than the unpromoted version. Promotion of the Cu/ZnO substrate was done by impregnation with an aqueous solution of K.sub.2 CO.sub.3. The isobutanol selectivity was maximized at 0.5 wt % K.sub.2 CO.sub.3.
Nunan, et al. (Journal of Catalysis, Vol. 116, pp. 195-221, 1989) describe a cesium-promoted Cu/ZnO catalyst for the selective synthesis of higher alcohols from synthesis gas. This catalyst was formed by calcination of a copper and zinc hydroxycarbonate precursor, followed by impregnation with an aqueous solution of cesium formate (CsOOCH). The optimum catalysts, in terms of C.sub.2+ alcohols yield, contained 0.3-0.5 mol % CsOOCH.
The above catalyst examples were used in a packed bed reactor for synthesis gas conversion. References to the selective synthesis of higher alcohols; using a slurry reactor are limited, but the following examples are worth noting.
European Patent Application EP-353920-A describes a slurry reactor based process for the production of mixed alcohols, with emphasis on the production of C.sub.2 -C.sub.6 alcohols. The catalyst used was a potassium-promoted cobalt and molybdenum on an alumina support. Promotion with potassium was done by impregnation with an aqueous solution of an alkali salt.
R. P. Underwood (Topical Report, Task 3.2 and 3.3, U.S. DOE Contract No. DE-AC22-90PC89865, December 1989-February 1993) describes slurry phase synthesis of methanol and higher alcohols from synthesis gas. A Cu/ZnO/Al.sub.2 O.sub.3 methanol synthesis catalyst, which had been promoted with cesium, was used together with mineral oil in a stirred autoclave. Promotion by cesium was done by impregnation of the Cu/ZnO/Al.sub.2 O.sub.3 substrate with aqueous cesium formate, followed by drying and calcination.
Chaumette, et al. (industrial and Engineering Chemistry Research, Vol. 33, pp. 1460-1467, 1994) describe the use of alkali-promoted Cu/Co/Zn/Al based higher alcohols synthesis catalysts in a slurry reactor. Alkalization of the catalysts was done either during the precipitation step or by impregnation of the calcined precipitate.
A distinguishing feature of the above examples, which are representative of the art, is that the catalysts were promoted with alkali metal compounds by a separate impregnation step. The impregnation step involves treating the catalyst with a known quantity of a solution of an alkali compound, followed by removal of the solute by drying. In all cases, alkalization of the catalyst is production step which is done outside of the reactor used by synthesis gas conversion.
Alkalization of a catalyst in the same slurry reactor which is used for synthesis gas conversion is not taught in the art for catalysts for the synthesis of higher alcohols from synthesis gas. However, in situ alkalization is taught in the art for Fischer-Tropsch catalysts, which are used for the production of paraffinic hydrocarbons from synthesis gas.
U.S. Pat. No. 2,671,103 by Kolbel et al. pertains to a slurry reactor process for the synthesis of paraffinic hydrocarbons from synthesis gas. In the subject process, an alkali-promoted catalyst (Fe, Co, Ni, or Ru-based) is suspended in a hydrocarbon oil product. The alkali promoter enhances the selectivity to C.sub.3+ hydrocarbons. To compensate for the loss in catalyst activity with time, catalyst is withdrawn from the reactor and fresh catalyst is added. With the fresh catalyst, a predetermined quantity of alkali is also added to make up for alkali lost in removing slurry from the reactor. The following compounds of sodium and potassium are mentioned as suitable alkali compounds: oxides, hydroxides, carbonates, hydrocarbonates, phosphates, silicates, borates, formates, acetates, and the salts of higher organic acids (soaps).
British Patent 708,744 by Kolbel et al. describes a method of alkalization of an iron-based Fischer-Tropsch catalyst in a packed bed reactor to maintain its wax-forming capability. In the described method, alkali metal compound is added to an extraction agent which is used for the periodic extraction of paraffin wax from the catalyst. In the preferred mode of operation, the alkali metal compound is added to the extraction agent after the bulk of the wax has been extracted. Suitable inorganic alkali metal compounds are identified in the patent as the oxides, hydroxides, carbonates, bicarbonates, phosphates, silicates, and borates of sodium and potassium. Also mentioned as being suitable are organic alkali metal compounds such as alcoholates, formates, acetates, or the alkali metal salts of higher organic acids. The patent states that the alkali metal compound, unless it is soluble in the extraction agent, must be finely ground and able to form a stable suspension in the extraction agent.
Frame and Gala (Proceedings of the U.S. Department of Energy Contractors Review Meeting, pp. 911-942, September 1993) presented the results of a laboratory investigation into the alkalization of an iron-based Fischer-Tropsch catalyst in a slurry reactor. Alkalization was done using potassium laureate (CH.sub.3 (CH.sub.2).sub.10 CO.sub.2 --K.sup.+) which is soluble in the hydrocarbon oil reaction medium. Alkalization was done at the start of CO hydrogenation or during an experimental run.